Step-down and step-up AC choppers are known as power conversion apparatuses that step down or up the voltage of an AC power source and output the resultant AC voltage. FIG. 11 is an explanatory drawing showing a single-phase step-down AC chopper disclosed in Patent Document 1 (identified below).
In the figure, the reference numeral 1 stands for a single-phase AC power source, 2 and 3—bidirectional switches, and 4—a load.
The bidirectional switch 2 is a circuit in which switching elements S1, S2, each having diodes connected reversely in parallel, are connected reversely in series. The bidirectional switch 3 is a circuit in which switching elements S3, S4, each having diodes connected reversely in parallel, are connected reversely in series. The bidirectional switch 2 and the bidirectional switch 3 are connected in series and connected to both terminals of the single-phase AC power source 1. In the load 4, a resistor R and a reactor L are connected in series. The load 4 is connected to both terminals of the bidirectional switch 3.
In this configuration, the bidirectional switch 2 and the bidirectional switch 3 are repeatedly and alternately ON/OFF operated on the basis of pulse-width-modulated gate signals. The gate signals of the bidirectional switch 2 and the bidirectional switch 3 are provided with a dead time such that prevents the two bidirectional switches from being simultaneously switched ON at an ON/OFF switching timing thereof. With such operation, the predetermined AC voltage obtained by stepping down the voltage of the AC power source 1 is applied to the load 4.
FIG. 12 is an explanatory drawing showing the single-phase step-up AC chopper disclosed in Patent Document 1.
In the figure, the reference numeral 1 stands for a single-phase AC power source, 2 and 3—bidirectional switches, 4—a load, 5—a reactor, 6—a capacitor.
The configuration of the bidirectional switch 2 and the bidirectional switch 3 is the same as in the above-described step-down AC chopper. The serial circuit of the reactor 5 and the bidirectional switch 2 is connected to both terminals of the AC power source 1. The serial circuit of the bidirectional switch 3 and the capacitor 6 is connected to both terminals of the bidirectional switch 2. The load 4 is connected to both terminals of the capacitor 6.
In such a configuration, the bidirectional switch 2 and the bidirectional switch 3 are repeatedly and alternately ON/OFF operated on the basis of pulse-width-modulated gate signals. A dead time is provided to the gate signals of the bidirectional switch 2 and the bidirectional switch 3 in order to prevent the two switching elements from being simultaneously switched ON at the ON/OFF switching timing thereof. With such operation, the predetermined AC voltage obtained by stepping up the voltage of the AC power source 1 is applied to the load 4.
However, in the case of a step-down AC chopper, where the bidirectional switch 2 and the bidirectional switch 3 are simultaneously switched OFF in the dead time, the path for releasing the inductive energy accumulated in the reactor L is cut off. At this time, a surge voltage is generated at both terminals of the reactor L. In the case of a step-up AC chopper, where the bidirectional switch 2 and the bidirectional switch 3 are simultaneously switched OFF in the dead time, the path for releasing the inductive energy accumulated in the reactor 5 is cut off. At this time, a surge voltage is generated at both terminals of the reactor 5. The surge voltage generated in the reactor L or reactor 5 can lead to the breakdown of the bidirectional switches 2 and 3.
Patent Document 1 discloses a method for inhibiting the occurrence of such surge voltage. According to Patent Document 1, three periods are provided for individually ON/OFF switching the switching elements S1 to S4, which constitute the bidirectional switches 2 and 3, according to the polarity of a load current Io or power source current Iin when the bidirectional switches 2 and 3 are ON/OFF switched. Each of those three periods can constitute a release path for the inductive energy accumulated in the reactor L or reactor 5. As a result, the occurrence of the surge voltage is inhibited.
Patent Document 1: Japanese Patent Application Publication No. 2003-230277.